Radio-over-fibre, RoF, technology is a promising solution where an optical network is used to federate several radio antennas in a communications network, making use of the large bandwidth offered by the optical fibre and its low loss. RoF is a technique that modulates radio frequency, RF, signals on optical signals for transmission over optical fibre. RoF technology is being considered for use in future generations of wireless radio communication systems to provide high quality broadband service to high density and mobile users. RoF is currently most widely used in the digital domain, particularly for the common public radio interface, CPRI. By using RoF it is possible to simplify the transceivers between fibre and radio and as a result significantly reduce costs.
Within an optical fibre point-to-point link connecting an antenna base station to a central office, the received RF wireless signals must undergo electrical-to-optical conversion, typically by direct modulation of a laser or through external modulation using an electro-optic modulator in conjunction with an optical carrier signal. In a technique known as subcarrier multiplexing, SCM, a plurality of RF signals are multiplexed and then modulated onto an optical carrier. Due to the nonlinear characteristic of electro-optic modulators, RF signals are typically weakly modulated onto the optical carrier, resulting in very low modulation efficiency. Consequently the power of the optically modulated RF signal can be more than 20 dB below that of the optical carrier. In addition, the nonlinear characteristics of the electro-optical modulator results in the generation of intermodulation products between the RF signals and the optical carrier, and between the optical subcarriers, which can cause further signal degradation.
In wireless communications network fronthaul, which connects radio equipment to radio equipment controllers, digital RoF techniques are mostly used, but analogue RoF techniques bring the advantage of lower latency and lower power consumption. As a drawback, in RoF, the overall system performance is limited by the performance of the optical subsystem blocks, primarily due to the nonlinear transfer function of the electro-optical modulator used to modulate each SCM signal onto its optical carrier signal. To improve the modulation efficiency, the optical power of the optical carrier signals can be increased by using a high power optical source or an optical amplifier. However, this may lead to increased intermodulation distortions at the receiver or even receiver damage due to the high optical power incident on the optical detector. Also, optical amplification increases the costs, and it is usually not acceptable for RoF applications.
Various analogue RoF techniques have been proposed including Brillouin scattering, external optical filtering, and optical attenuation but all of these techniques suffer intermodulation distortions, which are a particular problem for SCM RF signals. To reduce intermodulation distortion between the RF signals and the optical carrier, carrier suppressed modulation techniques may be used, as reported in C. Lim et al, “Investigation of Intermodulation Distortion Reduction Technique for Multi-Channel Fiber-Radio Transmission in Heterogeneous Access Networks”, Proc. LEOS 2006, but this approach does not mitigate the effect of intermodulation products between the optical subcarriers.